Negatively Charged Amino Acids within the Intraluminal Loop of Ryanodine Receptor Are Involved in the Interaction with Triadin

In mammalian striated muscles, ryanodine receptor (RyR), triadin, junctin, and calsequestrin form a quaternary complex in the lumen of sarcoplasmic reticulum. Such intermolecular interactions contribute not only to the passive buffering of sarcoplasmic reticulum luminal Ca²⁺, but also to the active Ca²⁺ release process during excitation-contraction coupling. Here we tested the hypothesis that specific charged amino acids within the luminal portion of RyR mediate its direct interaction with triadin. Using in vitro binding assay and site-directed mutagenesis, we found that the second intraluminal loop of the skeletal muscle RyR1 (amino acids 4860-4917), but not the first intraluminal loop of RyR1 (amino acids 4581-4640) could bind triadin. Specifically, three negatively charged residues Asp⁴⁸⁷⁸, Asp ⁴⁹⁰⁷, and Glu⁴⁹⁰⁸ appear to be critical for the association with triadin. Using deletional approaches, we showed that a KEKE motif of triadin (amino acids 200-232) is essential for the binding to RyR1. Because the second intraluminal loop of RyR has been previously shown to contain the ion-conducting pore as well as the selectivity filter of the Ca ²⁺ release channel, and Asp⁴⁸⁷⁸, Asp⁴⁹⁰⁷, and Glu⁴⁹⁰⁸ residues are predicted to locate at the periphery of the pore assembly of the channel, our data suggest that a physical interaction between RyR1 and triadin could play an active role in the overall Ca ²⁺ release process of excitation-contraction coupling in muscle cells.